Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have been proposed for use in power consumers such as vehicles as a replacement for internal combustion engines, for example. Such a system is disclosed in commonly owned U.S. patent application Ser. No. 10/418,536, hereby incorporated herein by reference in its entirety. Fuel cell systems may also be used as stationary electric power plants in buildings and residences, as portable power in video cameras, computers, and the like. Typically, the fuel cell systems generate electricity used to charge batteries or to provide power for an electric motor.
Fuel cells are electrochemical devices which directly combine a fuel such as hydrogen and an oxidant such as oxygen to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to result in the formation of water. Other fuels can be used such as natural gas, methanol, gasoline, and coal-derived synthetic fuels, for example.
The basic process employed by a fuel cell system is efficient, substantially pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators), and may be constructed to leave only heat and water as by-products. The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells, depending upon the context in which it is used. The plurality of cells is typically bundled together and arranged to form a stack, with the plurality of cells commonly arranged in electrical series. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a desired energy output level providing flexibility of design for different applications.
Different fuel cell types can be provided such as phosphoric acid, alkaline, molten carbonate, solid oxide, and proton exchange membrane (PEM), for example. The basic components of a PEM-type fuel cell are two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin catalyst layer. The electrodes, catalyst, and membrane together form a membrane electrode assembly (MEA).
In a typical PEM-type fuel cell, the MEA is sandwiched between “anode” and “cathode” diffusion media (hereinafter “DM's”) or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. The DM's serve as the primary current collectors for the anode and cathode, as well as provide mechanical support for the MEA. Alternatively, the DM may contain the catalyst layer and be in contact with the membrane. The DM's and MEA are pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct current between adjacent cells internally of the stack in the case of bipolar plates and conduct current externally of the stack in the case of monopolar plates at the end of the stack.
The secondary current collector plates each contain at least one active region that distributes the gaseous reactants over the major faces of the anode and cathode. These active regions, also known as flow fields, typically include a plurality of lands which engage the primary current collector and define a plurality of grooves or flow channels therebetween. The channels supply the hydrogen and the oxygen to the electrodes on either side of the PEM. In particular, the hydrogen flows through the channels to the anode where the catalyst promotes separation into protons and electrons. On the opposite side of the PEM, the oxygen flows through the channels to the cathode where the oxygen attracts the protons through the PEM. The electrons are captured as useful energy through an external circuit and are combined with the protons and oxygen to produce water vapor at the cathode side.
Many fuel cells use internal membranes, such as the PEM type fuel cell which includes proton exchange membranes, also referred to as polymer electrolyte membranes. In order to perform within a desired efficiency range, it is desirable to maintain the membranes in a moist condition. Therefore, it is necessary to provide a means for maintaining the fuel cell membranes in the moist condition. This helps avoid damage to or a shortened life of the membranes, as well as to maintain the desired efficiency of operation. For example, lower water content of the membrane leads to a higher proton conduction resistance, thus resulting in a higher ohmic voltage loss. The humidification of the feed gases, in particular at the cathode inlet, is desirable in order to maintain sufficient water content in the membrane. Humidification in a fuel cell is discussed in commonly owned U.S. Pat. No. 7,036,466 to Goebel et al.; commonly owned U.S. patent application Ser. No. 10/912,298 to Sennoun et al.; and commonly owned U.S. patent application Ser. No. 11/087,911 to Forte, each of which is hereby incorporated herein by reference in its entirety.
To maintain a desired moisture level, an air humidifier is frequently used to humidify the air stream used in the fuel cell. The air humidifier normally consists of a round or box type air humidification module that is installed into a housing of the air humidifier. Examples of this type of air humidifier are shown and described in U.S. Pat. No. 7,156,379 to Tanihara et al., hereby incorporated herein by reference in its entirety, and U.S. Pat. No. 6,471,195, hereby incorporated herein by reference in its entirety.
Membrane humidifiers, such as water vapor transfer (WVT) units, have also been utilized to fulfill fuel cell humidification requirements. For the automotive fuel cell humidification application, such a membrane humidifier needs to be compact, exhibit low pressure drop, and have high performance characteristics. Typical membrane humidifiers include a wet plate that includes a plurality of flow channels formed therein adjacent a DM. The flow channels are adapted to convey a wet fluid from the cathode of the fuel cell to the exhaust. Typical membrane humidifiers also include a dry plate that includes a plurality of flow channels formed therein adjacent a DM. The flow channels are adapted to convey a dry fluid from a source of gas to the cathode of the fuel cell. A similar membrane humidifier assembly can be used for an anode side of the fuel cell, or otherwise as desired. To avoid a leak of reactant gas from the wet side to the dry side, the membrane humidifier must be adequately sealed. A loss of reactant gas from the wet side to the dry side or from the dry side to the wet side will affect the humidification level of the reactants flowing through the membrane humidifier, as well as the stoichiometry of the reactants flowing through the fuel cell stack.
The flow channels are typically formed on both sides of the wet plate and the dry plate, which are separated by a web. The web militates against the deformation of the material forming the flow channels. By utilizing the web for support of the plates and flow channels, the overall dimensions of the plates and the flow channels are increased, thereby resulting in increased material and manufacturing costs, as well as an increased fabrication time of the membrane humidifier.
As the overall dimensions of the plates of the membrane humidifier increase, the thermal mass of the membrane humidifier increases. An increase in thermal mass results in an increase in a warm-up time during a starting operation of a fuel cell powered vehicle. During the starting operation, the membrane humidifier is typically warmed by only a flow of cathode reactant. Thus, as the thermal mass of the membrane humidifier increases, the amount of cathode reactant required to warm the membrane humidifier increases.
It would be desirable to a membrane humidifier, wherein the dimensions of the humidifier, the material costs of the membrane humidifier, and the assembly time of the membrane humidifier are minimized.